1. Field of the Invention
The present invention relates to image forming apparatuses such as copying machines and printers.
2. Description of the Related Art
In some image forming apparatuses, the image forming position can be adjusted in a direction perpendicular to the sheet conveying direction so that an image forming portion can form an image in the center in the width direction of a sheet. In these image forming apparatuses, the position in a direction perpendicular to the conveying direction of a sheet coming to the image forming portion is detected before image formation on the sheet by a sheet position detecting device installed in the main body of the image forming apparatus, and on the basis of the detection information, the image forming position is adjusted. By thus adjusting the image forming position according to the position of the coming sheet, an image can be formed at an appropriate position relative to the sheet.
In recent years, color image forming apparatuses employing various image forming technologies have been proposed. One of the technologies is an image forming technology using an intermediate transfer belt in an electrophotography technology. In this technology, an image unit is provided for each color of developer. In these image units, their respective colors of toner images are formed on their respective photosensitive drums through a known image forming process. These toner images are transferred (primary-transferred) sequentially onto an intermediate transfer belt. The toner images transferred onto the intermediate transfer belt 31 are transferred (secondary-transferred) together onto a coming sheet. The toner images transferred onto the sheet are fixed in a fixing device. In this way, image formation is performed on the sheet. In this type of image forming apparatus, the image units sequentially form their respective colors of images, which are superposed over one another and thereafter transferred onto a sheet. Therefore, the productivity of image formation can be improved.
FIG. 9 shows a color image forming apparatus employing this image forming technology using an intermediate transfer belt. The structure and image forming operation thereof will be briefly described. This color image forming apparatus includes an image unit 10, a paper feed unit 20, an intermediate transfer unit 30, and a fixing unit 40.
The image unit 10 includes four structurally identical stations, which are tandemly arranged. The stations include photosensitive drums 11a, 11b, 11c, and 11d, respectively, and primary charging devices 12a, 12b, 12c, and 12d, respectively. In addition, the stations include optical systems 13a, 13b, 13c, and 13d, respectively, developing devices 14a, 14b, 14c, and 14d, respectively, and cleaning devices 15a, 15b, 15c, and 15d, respectively.
The photosensitive drums 11a to 11d, which serve as image bearing members, are rotationally driven in the directions of arrows in the figure. The intermediate transfer unit 30 has an intermediate transfer belt 31, which serves as an intermediate transfer member. The upper portion of the intermediate transfer belt 31 forms a primary transfer plane A that can come into contact with the photosensitive drums 11a to 11d. As for the lower portion of the intermediate transfer belt 31, a secondary transfer internal roller 34 and a secondary transfer device 36 are disposed opposite each other with the intermediate transfer belt 31 therebetween and form a secondary transfer region Te.
The operation of this structure will be outlined. The primary charging devices 12a to 12d uniformly charge the surfaces of the photosensitive drums 11a to 11d, respectively. The photosensitive drums 11a to 11d are irradiated with and exposed to laser beams that are modulated according to a record image signal by the optical systems 13a to 13d. In this way, electrostatic latent images are formed on the photosensitive drums 11a to 11d. The electrostatic latent images are visualized into toner images on the surfaces of the photosensitive drums 11a to 11d by the developing devices 14a to 14d, which store yellow, cyan, magenta, and black developers (toners), respectively.
Thereafter, the toner images visualized on the surfaces of the photosensitive drums 11a to 11d are transferred onto the intermediate transfer belt 31 in primary transfer regions Ta, Tb, Tc, and Td. Toner not transferred onto a sheet P but left on the photosensitive drums 11a to 11d is scraped off by the cleaning devices 15a, 15b, 15c, and 15d. In this way, the surfaces of the photosensitive drums 11a to 11d are cleaned.
The toner images on the surfaces of the photosensitive drums 11a to 11d are transferred (primary-transferred) sequentially onto the intermediate transfer belt 31 rotating in the direction of arrow B, and the toner images of each color are superposed over one another on the intermediate transfer belt 31. In timed relationship with the image formation in the image unit 10, the sheet P sent out from the paper cassette 21a or 21b of the paper feed unit 20 is conveyed to the secondary transfer region Te, and the toner images on the intermediate transfer belt 31 are transferred (secondary-transferred) together onto the coming sheet. The toner images transferred onto the sheet P are fixed in the fixing unit 40. The sheet P is ejected by an external eject roller 45 onto a paper output tray 48.
In this color image forming apparatus employing the image forming technology using an intermediate transfer belt, when images are formed on both sides of a sheet, a sheet on a first side of which an image is formed is reversed by the external eject roller 45 serving as a reversing portion and is again conveyed to the secondary transfer region Te via a double-sided path 240. Thus, an image can also be formed on a second side of the sheet (the first side on which an image is formed will be referred to as first side, and the other side will be referred to as second side). In this way, images are formed on both sides of the sheet.
For example, when a plurality of sheets on both sides of which images are formed are bound to make a book, a significant difference in the position of images relative to each sheet between one side and the other side deteriorates the quality. Therefore, it is desired that the positions of images on the first and second sides relative to each sheet are identical to each other.
However, when images are formed on both sides of a sheet, the positions of the side edges (both edges along the sheet conveying direction) of the sheet being conveyed to the secondary transfer region Te to form an image on the second side of the sheet are significantly affected by the sheet reversing operation of the external eject roller 45. That is, the positions of the side edges of the sheet after the reversal tend to differ from those before the reversal. Therefore, it is necessary to detect the positions of the side edges (hereinafter referred to as side edge positions) of the sheet after the reversal and to adjust, on the basis of the detection, the position of an image to be formed on the sheet in a direction perpendicular to the sheet conveying direction. That is, after an image is formed on the first side and before an image is formed on the second side, the image position is adjusted by detecting the side edge positions of the sheet, obtaining the amount of deviation from reference positions, and, according to this amount of deviation, changing the timing of irradiation to the photosensitive drums 11a to 11d by the optical systems 13a to 13d. The term “changing the timing of irradiation” is defined as changing the irradiation start positions on the photosensitive drums 11a to 11d in the main scanning direction from which irradiation of laser light from the optical systems 13a to 13d is started.
However, in the image forming technology using an intermediate transfer belt, the distance from the irradiation position where the optical system 13d irradiates the photosensitive drum 11d to the secondary transfer region Te is long, and therefore the image forming process operation from the start of image formation to the transfer (secondary transfer) of toner images onto a sheet takes long time. Therefore, when the side edge positions of a sheet on the second side of which an image is to be formed are detected before the secondary transfer region Te and thereafter the image position in the image forming portion is adjusted, the sheet needs to be kept stopped for a long time until the transfer. Therefore, the productivity in image formation (the number of images formed on sheets per unit time) is significantly reduced. Therefore, it is necessary to detect as soon as possible the side edge positions of a sheet on the second side of which an image is to be formed, in order to start as soon as possible the image forming operation. For this purpose, as shown in FIG. 9, an edge detecting sensor 800 for detecting the side edge positions of a sheet on the second side of which an image is to be formed is provided downstream of the reversing portion and upstream of the double-sided path 240, thereby improving the productivity during two-sided image formation. This technology is disclosed in Japanese Patent Laid-Open No. 2002-80144.
However, in recent years, it has been strongly desired that image forming apparatuses have high productivity. In the above-described image forming apparatus, the edges of a sheet are detected by the edge detecting sensor 800 provided in the double-sided path 240, and thereafter adjustment of the image position in the image forming portion is started. Therefore, the productivity can be improved to some extent but is not sufficient.
That is, the edge detecting sensor 800 needs to be provided downstream, in the sheet conveying direction, of the sheet reversing portion, which significantly affects the position of the sheet. On the basis of the detection, the image forming operation in the image forming portion is started. Therefore, the productivity of image formation is significantly limited by the position where the edge detecting sensor 800 is disposed. Therefore, conventional apparatuses cannot sufficiently meet the demands for high productivity.